[Xen-devel] Re: [PATCH]Add a flag for shadow pages

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

I just realised that. You use get_page() to lock down a page's owner.
Otherwise it can change under your feet anyway. You don't need a new flag.

 -- Keir

On 04/03/2009 09:17, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote:

> This is mainly for page offline. When we offline a page, we we need turn to
> the caller the page's owner but we can't distinguish easily if it is a
> assigned page or a shadow page.
> 
> Thanks
> Yunhong Jiang
> 
> Keir Fraser <mailto:keir.fraser@xxxxxxxxxxxxx> wrote:
>> On 04/03/2009 08:46, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx> wrote:
>> 
>>> Currently we don't know that a page is a shadow page unless we are in
>>> shadow handler. This cause error when we try to get the page owner for the
>>> shadow page, this snippet add a flag to it.
>>> 
>>> signed-off-by: Jiang, Yunhong <yunhong.jiang@xxxxxxxxx>
>> 
>> You only actually use your new flag in a BUG_ON, where it
>> might avoid false
>> negatives but hardly looks essential. Your other changes limit count_info
>> checks to PGC_page_table|PGC_count_info which might be an
>> improvement (I'm
>> sure Tim or Gianluca can say) but they're existing fields.
>> 
>>> I'm not quite sure if the sh_put_ref() and
>> sh_rm_write_access_from_sl1p() is
>>> try to checking a page is shadow page (I assume so), because when a
>>> anonymous page is allocated, the count_info is also 0 (like HVM's vlapic
>>> page), so I change it like this patch (checking PGC_count_mask is 0).
>>> Since comments in sh_hash_audit_bucket() has stated clearly it is to check
>>> if it is shdow, so I replace it with test_bit().
>>> 
>>> Also, do we need checking in page_get_owner() also?
>> 
>> Usually gets used where we already get_page()ed or are about
>> to get_page().
>> Most Xen code knows that page uses can change under its feet
>> unless it grabs
>> a reference. So extra checks in page_get_owner() are probably rather
>> pointless. 
>> 
>> -- Keir



_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel